The present invention relates to papermaking machine elements which are loaded to establish a pressure profile in general and in particular to systems for measuring the pressure profile generated by the element.
In any part of a papermaking machine or paper processing machine, such as a press or calender, where a paper web is passed through a nip formed between two rolls or between a shoe and a backing roll, it is desirable to control the nip pressure in the cross machine direction, and the nip pressure profile in the machine direction. Typically it is desirable to make the nip pressure as uniform as possible in the cross machine direction so that the entire web is uniformly treated. The pressure profile in the machine direction is tailored to achieve optimal results in calendaring or dewatering, or to minimize wear. A uniform nip loading can be accomplished by employing a crowned roll; however, for a given amount of crown, only a single uniform nip loading can be achieved. Oftentimes, as the grade of paper or type of paper changes it is desirable to change the nip loading. Further, as the width of papermaking machines has been increased to as much as ten meters it has become more difficult to achieve uniform loading along the entire cross machine direction length of the nip using simple crowned rolls.
The solution is to use a deflection-compensated roll, sometimes also referred to as a crown control roll. In a deflection-compensated roll, one or more hydraulic pistons is arranged in the cross machine direction within a roll shell. The hydraulic pistons are mounted to a roll support beam. The hydraulic pistons form hydraulic loading devices which directly support the shell by applying load to the inside surface of the roll shell. Deflection-compensated rolls are typically employed in the pressing section of a papermaking machine, or in the rolls of a calender or supercalender. Deflection-compensated rolls require a high capital investment and, to gain full benefit from the investment, it is desirable to control each deflection-compensated roll as accurately as possible.
Deflection-compensated rolls can be divided into rolls with a mobile shell and rolls with a fixed shell. Both types of deflection-compensated roll have a nonrotating support beam about which the roll shell is mounted. The shell may be fixed with respect to the support beam by end bearings which may be roller bearings or hydraulic slide bearings. To close the nip between a deflection-compensated roll which is fixed with respect to a support beam, the entire support beam is moved onto loading arms to close the nip. Alternatively, the roll shell may be radially movable on the support beam with respect to the axis of the shell in the plane defined by the nip and the roll axis so that the nip may be opened and closed by such radial movement.
To determine the cross machine nip load, in a deflection-compensated roll where the roll shell is fixed with respect to the support beam, the total loading of the deflection-compensated roll may be determined by determining the loading of the support beam. Alternatively, or in addition, the loading applied to the roll shell by loads in the end bearings, and in the individual cross machine direction loading devices, can be determined, typically by measuring the hydraulic pressure supplied to the loading devices and end bearings, and calculating load based on the cross-sectional area of the pistons or bearings. Where the roll shell is mounted for radial motion, the nip loads can be determined from the total hydraulic loading of the cross machine direction loading devices.
The pressure in a nip between two rolls has been measured directly by pressure sensors which are placed in the nip as described in U.S. Pat. No. 5,953,230, or by a process known as NipScan® developed by Albany International Corp. of Albany, N.Y. Determining nip pressure or roll bearing loads by any of the foregoing techniques is important because paper quality can depend on nip uniformity. In addition, excess nip loading can damage elastic roll covers which are used to provide a so-called “soft nip”. What is needed is a better technique and system for characterizing the nip load formed between two rolls, or between a shoe and a backing roll.